Megathrust quake risk real, if remote

Just when you thought it was safe to go back to the water, an American geophysicist has built a computer model of a ‘‘megathrust’’ earthquake which would send tsunami waves 12 metres high crashing against the east coast from Hawke’s Bay to Christchurch.

Steven Ward from the University of California has been studying the Hikurangi subduction zone, running offshore north-east from Marlborough, where the Pacific and Australian tectonic plates collide.

Ward’s computer simulation of a very large offshore earthquake demonstrates how tsunami could engulf the eastern shorelines of both the North and South Islands, with little warning – from 10 minutes to an hour. Reporting of Ward’s work has been decried on social media as alarmist and scaremongering. Essentially, it is also old news.

Surveys by the underwater research vessel Tangaroa from 2011 revealed evidence of more than 150 quakeinduced submarine landslides and many active faults in a canyon in Cook Strait, just 10km off the Wellington coast. Geologists since the 1990s have been telling us that a series of concentric former beachline terraces at Turakirae Head, south-east of Wellington, also provide evidence of previous ‘‘uplift’’ earthquakes, strong enough to lift land out of the sea.

Radiocarbon methods suggest there have been at least four such events in the last 7000 years. That may seem like a reassuringly long time, except that the most recent two happened in only the last 600 years. One of them is well documented – the magnitude 8.2 Wairarapa earthquake of 1855 which dashed buildings to the ground in the young settlement of Wellington, raised land out of the sea along its waterfront, and sent tidal waves over Miramar Peninsula. At Palliser Bay, the tsunami was 9 metres high and washed away buildings and boats.

There is evidence of a much earlier massive earthquake or series of events which possibly gave rise to the legend of Hao-Whenua, or ‘‘the land swallower’’.

The story of Hao-Whenua was told to the early 20th century Pa¯ keha¯ ethnologist Elsdon Best by Wairarapa Ma¯ ori, who informed him there were formerly two entrances to Wellington Harbour – the current entrance and one where the suburb of Kilbirnie now stands.

Best was sceptical of the story because he could not fathom how an earthquake which lifted the seabed could be called a ‘‘land swallower’’. He apparently did not consider that the name might refer to the subsequent tidal waves which swept in from the sea. Best reckoned Hao-Whenua came 18 generations before his time, which would coincide with archaeological and other evidence which suggests that many coastal dwellers moved inland in the middle of the 15th century.

In modern times, while there is no immediate reason to be alarmed, no-one should turn a blind eye to the deadly potential of an offshore quake which, at possibly magnitude 9, could be much stronger than the Kaiko¯ ura earthquake of 2016.

And no-one should blame scientists, or accuse them of being alarmist, when they simply tell what they can see in the evidence before them. If we choose to live in this dynamic and unpredictable environment, we need to be aware both of its history and what it is capable of doing to us.

Scientists shouldn’t be blamed for outlining the evidence before them.